A Good Reason For Sex

Water fleas elucidate the evolution of gender.

The question of why organisms have sex may seem trivially easy—any mortified kid who's sat through a birds-and-bees lecture knows that it's to reproduce. But biologists have for many decades struggled to puzzle out an ironic quirk of sexual reproduction: since males cannot produce offspring, sexual species have only half the raw reproductive capacity of asexual ones, in which every individual can crank out the young'uns. Despite decades of research, biologists still have not pinned down the theory of how sexual reproduction—which seems to work out okay in the real world—makes up for this huge cost.

A study of the tiny water flea published in Science last week provides what might be the first direct evidence of why sex is so good—on a species level. The water flea was practically born for such research because it lives both in sexual groups and in asexual ones, which branched off from sexual groups at various times in the past. Two Indiana University biologists, Susanne Paland and Michael Lynch, looked at the genetic signatures of various flea populations from Illinois to Nova Scotia to see how the sexual and asexual groups differed.

Their research showed that after the asexual groups stopped reproducing sexually, they all began picking up negative mutations—mutations that hurt their owners in the natural-selection game—faster than their sexual counterparts. So although the loner fleas could reproduce faster, over time, their gene pools became less desirable than ones who never broke up with sex.

The researchers say this supports one of the leading theories of why sexual reproduction is so prevalent: the randomness of genetic recombinations can help get rid of harmful mutations. In an asexual species, any negative mutation in an organism's germ line is passed on to all of its descendants, which draw every one of their genes from the one ancestor. But in sexual organisms, there's a chance that a bum gene from one parent will get shuffled out of the mix in its offspring. Legendary geneticist John Maynard-Smith compared recombination to swapping the parts from two cars—one with a broken engine and one with a broken transmission—to make a smoothly working one.

Of course, this will also leave you with a real lemon: a car with a busted transmission and engine. And recombination, similarly, will produce some offspring with particularly bad sets of traits—even worse than the asexual organisms, the ones that in the car example are permanently stuck with one broken part. Many geneticists say that over time, the very best offspring of sexual reproduction will out-compete not only their inferior sexual relatives, but also the more distantly related asexual ones. Further studies of the helpful water flea and other organisms may show if this really explains why we humans are stuck with sex—and why our future descendants should be happy about it. —Amos Kenigsberg